U.S Pat. No. 3,168,553 discloses a process in which olefins are carboxylated in the presence of a complex comprising a trialkylphosphine together with cobalt, ruthenium, rhodium or iridium. However, this process requires the use of high pressures, and its selectivity towards the desired product is often unsatisfactory. For instance, the carbonylation of ethylene in the presence of ethanol and CO.sub.2 (CO).sub.8 as the catalyst leads to the formation not only of ethyl propionate, but also of large quantities of by-products, such as diethyl ketone and acetaldehyde.
J. Molecular Catalysis 40 (1987) 243-254 concerns the synthesis of ketones and esters from olefins, carbon monoxide and alcohols by using ruthenium-iodide catalysts. In the carbonylation of ethylene with methanol, use of ionic iodides such as sodium iodide gives methyl propionate selectivity, using pressures of carbon monoxide and ethylene of 45 and 20 kg/cm.sup.2, respectively.
It has now been found that by selecting a molar ratio of carbon monoxide to olefinically unsaturated compound below a certain value, the process results in a higher reaction rate and even in a higher selectivity to esters of carboxylic acids. Moreover, said known process can successfully be amended by using certain bromides.